Shading artifacts in cycles displacement
A couple of things come to my mind:

If it's really the image that's the problem, upscale and blur it beforehand. This will smooth any compression/resolution artifacts and could work in your case.
Are you using Adaptive Subdivision? If not, you might want to do
that. Go to Render Properties &gt; Feature Set &gt; Experimental. Then add
a Subdivision Surface modifier set to Simple and check Adaptive
Subdivision. In viewport you won't see the full resolution though. Also, make sure to use Displacement Only. Material Settings &gt; Settings &gt; Surface &gt; Displacement &gt;Displacement Only.
Try different interpolations in the Image Texture node.